More and more rigorous demands for characteristics of non-aqueous electrolytic solution for lithium secondary battery have been imposed year by year. One of such demands is to solve a problem that metal is precipitated in the form of dendrite on a protective film formed on a metal surface of a negative electrode, thereby causing lowering of rate characteristic and cycle characteristic of a battery in some cases, and in the worst case, resulting in a danger that dendrite reaches a positive electrode surface and causes short circuit and firing.
In order to solve such a problem, there is proposed a method of fluorinating ethylene carbonate being a good solvent for dissolving an electrolyte salt for inhibiting formation of dendrite and improving cycle characteristic (JP7-240232A, JP2003-168480A, JP2004-319317A, JP2007-188873A and JP8-306364A).
Fluorination of ethylene carbonate has been shifted from monofluorination to difluorination for enhancing ability of forming a film (protective film) (JP7-240232A, JP2003-168480A and JP2004-319317A).
However, difluoroethylene carbonate is instable and high in hydrolyzability and therefore, is difficult to use, and synthesis and refining of it is difficult. Also, there is little difference in ability of film formation between difluoroethylene carbonate and monofluoroethylene carbonate. Therefore, the use of difluoroethylene carbonate has not proceeded.
Fluorination of substituted ethylene carbonate is also disclosed (JP2007-188873A and JP8-306364A). In JP2007-188873A, a variety of general formulas are proposed and a lot of compounds are raised as examples thereof, but fluorinated ethylene carbonates actually used in examples are only monofluoroethylene carbonates and difluoroethylene carbonates similar to those of JP7-240232A, JP2003-168480A and JP2004-319317A.
In JP8-306364A, fluoroethylene carbonate having methyl as a substituent is disclosed and is said to have an effect of inhibiting formation of dendrite. However, even in JP8-306364A, fluorinated ethylene carbonate actually used in examples is only one which is 1-methyl-2,2-difluoroethylene carbonate. Also, fluorinated ethylene carbonate is contained in an amount of as much as not less than 10% by volume, preferably not less than 30% by volume.
Also, further enhancement of safety (for example, incombustibility and breaking resistance) at over-charging is demanded.